1. Field of the Invention
The invention is generally related to nuclear fuel rods and more particularly to a clad tube with a rhenium liner that is produced by an electro deposit process.
2. General Background
Numerous materials have been investigated for use as cladding for uranium nitride nuclear fuel. Near the end of the SP-100 space nuclear reactor program clad development was focused on a fuel pin design having a Nb (niobium)-1% Zr (zirconium) outer shell with a rhenium liner. The purpose of this design was two-fold. First was that the 1% Zr would be reduced by the free contaminant oxygen found in the niobium alloy. Second was that the rhenium also provided a boost in creep strength over an unlined Nb-1% Zr tube. The outer niobium alloy was generally a powder metallurgy fabricated tube that typically had trace (˜1,000 ppm) impurities of oxygen. Since lithium was to be the reactor coolant, any free oxygen was scavenged from the clad by the lithium. This led to pitting in the pure niobium material and, ultimately, to clad/fuel failure. It was found that by adding 1% Zr to the niobium, the zirconium would “tie up” the free oxygen such that the lithium could not getter it from the clad and, hence, not form pits in the clad. The inner rhenium liner protects the niobium alloy from chemical attach by the uranium nitride fuel.
The method of manufacture of the tubes during the SP-100 program was not direct and required subsequent forming to achieve a finished product. The basic process was a complicated forming of powder metallurgy rhenium sheet, followed by a multi-step rolling operation to form the rhenium into a tube shape. The tube seam was then EB (electron beam) welded along its length to form a closed tube. The rhenium tube was then slid into a close tolerance fit niobium tube and a specially developed braze/weld performed to join the rhenium and niobium. This resulting clad was then loaded with fuel and seal welded.